The present invention relates to a regeneration system for a particulate trap, and in particular to a system for controlling the trap temperature upon the regeneration of the particulate trap (hereinafter abbreviated as trap).
There is shown in FIG. 1 a conventionally known system for controlling the trap temperature upon the regeneration by means of an electric heater as a thermal source. It is to be noted that the regeneration of the trap means the reproduction of the trap by burning particulates (particles) accumulated in the trap while a vehicle is running.
In the figure, in an exhaust pipe 1 of an engine (not shown) is provided a well-known particulate trap 2 connected to the exhaust pipe 1 and having an inlet, at the front of which an electric heater 3 is installed. A switch relay 4 turns on a power source for the electric heater 3, a thermal sensor 5 is provided inside the trap 2 for sensing the temperature at the front of the trap 2, and an air pump 6 supplies air flow to the trap 2.
At the front of the trap 2 in the exhaust pipe 1 is provided a trap inlet valve 7 for controlling the flow of exhaust gas from the engine flowing into the trap 2, while in a bypass pipe 8 for bypassing the exhaust gas out of the trap 2 is provided a bypass valve 9 for controlling the flow of the exhaust gap into the bypass pipe 8. The inlet valve 7 and the bypass valve 9 are controlled by vacuum solenoid switch valves 10 and 11 respectively.
A pressure sensor 12 is provided for sensing the pressure difference between the front and rear of the trap 2, and a controller (CPU) 13 receives the sensed signal from the sensor 5 and supplies control signals to the relay 4 and the switching valves 7 and 9.
In the conventional operation by the controller 13, when it is decided from the output of the pressure sensor 12 that enough particulates are accumulated in the trap 2 for the regeneration time thereof, the bypass valve 9 is opened, the inlet valve 7 is closed, and the heater 3 is electrically energized by the switch relay 4 to raise the trap temperature up to a predetermined ignition temperature for the accumulated particulates.
Such an electrical heater used for raising the temperature of the trap front is generally of a sheath type, which has a rod-like shape (with a diameter of 5.about. 6 mm) as shown in FIG. 2, so that in the event of null air flow as shown in FIG. 2(a) the generated heat of the electric heater 3 is well conducted around the heater 3 but scarcely conducted far from the heater 3, with the result that the trapped particulates are non-uniformly burnt.
Then, in the event of air flow given as shown in FIG. 2(b), calories dissipated around the heater are carried onto the surface of the trap, resulting in good thermal conduction.
Therefore, when the front temperature of the trap reaches the particulate ignition temperature, the controller 13 initiates the supply of air flow by driving the air pump 6 for uniform and transmissible regeneration.
Such a system employing an air pump for transmissible regeneration is disclosed e.g. in Japanese Utility Model Application laid-open No. 62-49610 of this applicant, while there is another system introducing air at the same time as the commencement of the energization of the heater as disclosed in Japanese Patent Application laid-open No. 59-101518.
After this, when a predetermined time interval has lapsed from the energization of the heater 3, the air supply is stopped, the bypass valve 9 is closed, and the inlet valve 7 is opened, thereby reopening the ordinary trap operation.
In such a conventional regeneration operation for a particulate trap, the exhaust gas of the engine passes through the bypass pipe 8 during the regeneration time interval.
However, although the trap inlet valve 7 is closed, there is actually a gap, between the inlet valve 7 and the exhaust pipe 1, from which the exhaust gas leaks. Furthermore, the exhaust back pressure becomes high when the revolution (load) of the engine is high, so that the leak amount becomes large correspondingly, whereby the surfaces of the heater 3 and the trap 2 are cooled because of the addition of the introduced air amount from the air pump 6, disadvantageously resulting in a delayed thermal increase until the ignition of the particulates and in a worsened thermal efficiency.
On the other hand, if the introduced air amount from the air pump 6 is decreased to hasten the thermal increase unduly, the trap 2 per se will be melted.